The toxicology reports are in and now we know: Two dolphins in Switzerland died from a drug overdose.

Shadow and Chelmers, residents of the Connyland Marine Park in northeast Switzerland, died within five days of each other in November — soon after the area adjacent to their tank was rented out for a weekend rave. Initial speculation surmised that the loud music stressed the dolphins, leading to a weakened immune system and eventually death. But an autopsy revealed that a heroin substitute was the real culprit.

Toxicology reports from a forensics institute in St. Gallen, Switzerland, were just released and show the two dolphins had buprenorphine, a heroin substitute, in their urine, according to the Daily Mail. Reports speculate that the substance may have been dumped in the tank during the rave, accidentally or as a practical joke.

Experts believe the drug plays tricks on a dolphins’ brain, switching off the monitor telling them they need to surface for air; the effects certainly sound terrifying. Connyland dolphin keeper Nadja Gasser told the Daily Mail that trainers noticed Chelmers simply drifting underwater and in obvious trouble. “We tried to hold him. He was shaking all over and was foaming at the mouth,” she said. Trainers worked unsuccessfully for an hour in an attempt to rescue the dolphin.